Optical fiber has reached a high degree of development. However, some shortcomings remain in at least some fibers. Among these is a condition frequently referred to as "curl" or fiber "bow". See, for instance, U.S. Pat. No. 5, 284,499.
As described in the '499 patent, ". . . differential cooling of a drawn fiber before the viscosity of the cladding layer of the fiber is high enough to substantially prevent differential stresses in the drawn fiber may cause the fiber to bend. This bending, or fiber "bow", causes difficulty when the fiber is spliced to other fibers, resulting in high loss splices . . . ". The problem is most severe in mass fusion splices.
Fiber bow is usually expressed in terms of a deflection, or in terms of a radius of curvature. It can be measured by extending a short length (e.g., 18.9 mm) of uncoated fiber beyond a restraining fixture, rotating the fiber, and measuring the maximum deflection of the fiber end. The deflection measurement can, if desired, be converted into a radius of curvature using the approximate expression R=l.sup.2 /2.delta., where R is the radius of curvature, l is the length of fiber extending from the fixture, and .delta. is the fiber deflection, all in meters.
The '499 patent discloses apparatus for drawing optical fiber having reduced bow but does not disclose values of bow obtained by means of the apparatus. We have carried out experiments and have obtained bow of 10-20 .mu.m with apparatus of the type shown in '499.
In view of the importance of having fiber with low bow, it would be desirable to have available a method of making such fiber (e.g., a method of making fiber having a mean bow of 5 .mu.m or less). This application discloses such a method.